1. Field of the Invention
This invention relates generally to subsea well equipment and methods. In particular the invention relates to apparatus and methods for controlling subsea christmas tree functions during workover operations.
2. Description of the Prior Art
The conventional method of controlling underwater (subsea) tree functions has been through a connection method from a remote hydraulic or electrical/hydraulic source acting via a control or umbilical line and an interface plate(s). These interface plates have been disconnected and reconnected in various ways to switch remote operation from a production (or "host") facility to a vessel overhead during equipment installation and later workover (well intervention). The key to the interfaces is that when in the workover mode, the production mode of operation is locked out, thereby preventing accidental operation by outside sources when critical control of the well is required by the overhead vessel. FIGS. 1A-1D illustrate common practice methods to achieve this crucial requirement.
Prior Shallow Water Arrangements
FIGS. 1A and 1B illustrate that for shallow water depths, disconnect/reconnect operations employ a "stab plate" 2 as part of the shallow water tree 4 as shown in FIG. 1A. The shallow water tree 4 is secured by means of a connector 16 to a wellhead 14 which is secured to the seabed 16. A tree cap 18 closes the top of the tree 4. A conventional stab plate 2 is a junction plate which connects the production hydraulic umbilical 6 from the host remote production platform/production tree 8 to the shallow water tree 4. In other words, hydraulic power is directed to each of the valve actuators 10 of the shallow water tree 4 via the hydraulic lines of the hydraulic umbilical 6 via the conventional stab plate 2 connection.
When workover operations are required, as FIG. 1B illustrates, the production hydraulic umbilical 6 is removed (e.g., by a diver) and parked at a seabed parking plate 12. The shallow water tree cap 18 is removed, parked on the seabed 16, and a workover vessel 20 with a riser 22 and workover equipment is attached to the top of the shallow water tree 4. A workover production umbilical 24 is plugged into the hydraulic line receptacle 26 of the stab plate 2. The vessel 20 assumes control of the hydraulic actuators 10 of the shallow water tree 4.
Prior Deep Water Arrangements
FIGS. 1C and 1D illustrate prior art transfer of control from a platform/production tree 80 to a workover vessel 200 for workover operations. A wellhead 140 and tree master block 40 extend from seabed 16. FIG. 1C illustrates that a tree control pod 30 is placed adjacent a tree manifold 42 which is placed above the tree master valve block 40. A hydraulic production umbilical 6' is connected between the tree control pod 30 and the platform/production tree 80. Control over the flow of each hydraulic line of umbilical 6' is by means of an electrical control system in the tree control pod 30. Control signals are transmitted from host platform/production tree 80 via electrical umbilical 62. Each hydraulic line is connected in the tree manifold 42 by means of "U-loop" lines 46 in the tree cap 180.
FIG. 1D illustrates a prior art or "conventional" deep water workover operation. The tree cap 180 of FIG. 1C is removed (with its "U loop" routing paths) from the tree manifold 42, thereby removing all control of valve actuators 100 from the host remote production/platform tree 80. A completion riser tree running tool 44 replaces the tree cap 180. Conventionally, a Lower Marine Riser Package (LMRP) 47 is secured to the top of running tool 44 and an Emergency Disconnect Package (EDP) 48 is secured to the top of the LMRD. A workover umbilical 240 is provided from the workover vessel 200 to the running tool 44. During workover operations, tree valve actuators 100 are controlled directly from the workover vessel 200. In some cases an additional electro-hydraulic control pod 50 on the riser Emergency Disconnect Package 48 is provided for control of hydraulic actuator control paths.
After the well intervention is finished, the tree running tool 44 is removed and the tree cap 180 is replaced as in FIG. 1C. With the tree cap 180 back in place, control over the tree valve actuators 100 is assumed again by the remote host facility 80 (that is, by the platform/production tree 80). In other words, the "U-loops" 46 which communicate with the tree control pod 30 are again in place and provide control paths for hydraulic fluid to all valve actuators 100 of the tree 40.
To date, the "U-loop" tree cap 46 arrangement and method has been acceptable. However, the "U-loop" tree cap arrangement and method requires twice the number of porting connections for every subsea tree function. Ancillary technology functions (e.g., chemical injection, multiple zone completion--"smart well", etc.) are requiring more and more functions through the top of the tree (which requires doubled of the parting connections by the "U-loop" configuration). Packaging constraints, the degradation of reliability (because of the excessive number of ports, check valves, and leak paths) and manufacturing costs associated with assembling and testing the increasing number of lines makes the "U-loop" configuration more and more impractical.
Workover control systems have traditionally been entirely hydraulic, but they have been replaced recently with electro-hydraulic systems as the subsea oil and gas industry has been producing from deeper and deeper water depths. Deep well depths increase the size and cost of hydraulic control lines. Reels for the hydraulic control lines become too large to handle and/or response times to operate the subsea tree become too long.
Identification of Objects of the Invention
A primary object of the invention is to provide a deep water workover interface system which reduces costs associated with the "U-loop" tree caps of prior art deep water vertical production trees.
Another object of the invention is to increase the control functions available in a new workover interface arrangement for a subsea tree.
Another object of the invention is to improve the reliability of a deep water workover interface arrangement by (1) providing a functioning subsea control pod prior to demobilizing the host control, (2) eliminating potential hydraulic leak paths inherent in the current "U-loop" tree cap arrangement and (3) improving hydraulic testing characteristics of the arrangement during FAT/SIT and offshore testing time.
Another object of the invention is to reduce rig time required of current operations by eliminating a drill pipe trip to install the tree cap after workover operations are complete and production operations are to begin again.